Method of electrically welding a contact to a resistance wire

ABSTRACT

CLAIM 3. method of manufacturing electrical resistance elements comprising the steps of: WINDING A PLURALITY OF TURNS OF BARE, RESISTANCE-TYPE WIRE ONTO AN INSULATED CORE, SAID WIRE HAVING A PREDETERMINED MELTING POINT; FABRICATING A TERMINATION TAB FROM A MATERIAL HAVING A MELTING POINT SUBSTANTIALLY HIGHER THAN THE MELTING POINT OF SAID WIRE, SAID TAB BEING OF SUFFICIENT WIDTH TO SPAN A PLURALITY OF TURNS OF SAID WIRE; PLACING SAID TAB TRANSVERSELY ACROSS SAID CORE WITH A PORTION THEREOF IN INTIMATE CONTACT WITH A PLURALITY OF TURNS OF SAID WIRE; AND SUPPLYING ELECTRICAL ENERGY TO SAID TAB BY ELECTRODES AT OPPOSITE SIDES OF SAID PORTION TO HEAT SAID TAB AND FUSE SAID WIRE ONTO SAID TAB AT SAID PORTION BETWEEN SAID ELECTRODES.

United States Patent Inventors Jack E. Langenbach Corona Del Mar; James E. McAdoo, Santa Ana; Paul L. Pataky, Anaheim, Calif.

Appl. No. 473,266

Filed June 7, 1965 Division of Ser. No. 144,251, Oct. 10, 1961, abandoned.

Patented Apr. 6, 1971 Assignee Beckman Instruments, Inc.

METHOD OF ELECTRICALLY WELDING A CONTACT TO A RESISTANCE WIRE [5 6] References Cited UNITED STATES PATENTS 2,046,129 6/1936 Mucher 338/332X 2,890,320 6/1959 Oakley 338/329X 3,089,020 5/1963 l-Iurlebaus 219/86 Primary Examiner-R. F. Staubly Att0rneys-Robert J. Steinmeyer and Ferd L. Mehlhoff CLAIM: CLAIM 3. method of manufacturing electrical resistance elements comprising the steps of:

winding a plurality of turns of bare, resistance-type wire onto an insulated core, said wire having a predetermined melting point; fabricating a termination tab from a material, having a melting point substantially higher than the melting point of said wire, said tab being of sufficient width to span a plurality of turns of' said wire; placing said tab transversely across said core with a portion thereof in intimate contact with a plurality of turns of said wire; and supplying electrical energy to said tab by electrodes at opposite sides of said portion to heat said tab and fuse said wire onto said tab at said portion between said electrodes. "9 v v PATENTEDAPR 6|97| 3573, 122

SHEET 1 BF 2 mmvroxs JACK E. LANGENBACH BY amass. McADOO PAUL PATAKY ATTORNEY PATENTED APR 6 I97! SHEET 2 OF 2 FIG. 3 FIG. 4

42 (o) (b) (c) FIG. 5

IN V EN TORS JACK E. LANGENBACH JAMES E McADOO PAUL L. PATAKY ATTORNEY cal contact for variable resistance elements and a method for making such contact.

Variable resistance devices such as potentiometers, rheostats and the like require one or more fixed electrical contacts with the resistance element thereof. Primary considerations involved in constructing fixed electrical contacts for variable resistance devices are the physical location of the contact with respect to the resistance element, the structural configuration of the contact, and the method employed for attaching the contact to the resistance element. As will be seen below, each of these factors substantially influence the cost of the variable resistance device and the accuracy and precision obtainable therefrom. Moreover, each of the structures and techniques presently known have one or more disadvantages and limitations which obviate their universal application and, in particular, their application to miniature potentiometers.

The linearity of a variable resistance is determined by several factors, one of which is the physical location of the fixed electrical contact with respect to the resistance element. If, for example, the fixed contact is so positioned that there is an electrical resistance between the potentiometer wiper and the fixed electrical contact when the movable wiper is positioned mechanically to obtain the minimum resistance at the contact being measured, this resistance, known in the art as end resistance" or "residual resistance is a deviation from the desired linear output. The problem of end resistance is also influenced by the'structure of the fixed electrical contact since an electrical connection having appreciable resistance between the output terminal and the resistance element will add to the end resistance.

The limitations inherent in the prior art structures and methods for making fixed electrical contact with the resistance element of a variable resistance device may be illustrated with regard to a miniature trimmer potentiometer employing a resistance element comprising resistance wire helically wound upon a metal mandrel. Such a potentiometer, for example, is shown U. S. Pat. No. 3,099,8 l issued in the name of Hans Habereder and described in the entitled Miniature Potentiometer" filed on Mar. 18, I960 and assigned to Beckman Instruments, Inc. assignee of the present invention. This potentiometer is designed to occupy a very small space, e.g., the longest side to be not over gfl'nch with a l-inch long mandrel and have the same resistance value as a 2-inch diameter potentiometer with a 5-inch long mandrel. In order to manufacture a potentiometer which meets these specifications, the following four modifications may be considered:

1. Greater resistivity per unit length resistance wire;

2. A larger diameter mandrel;

3. A closer spacing of the resistance wire when wound on the mandrel; or

4. Smaller diameter resistance wire. Of these four altematives, the first is limited by the commercial unavailability of satisfactory resistance wire having a resistance of greater than 800 ohms-circular mils per foot. The second is limited by the thickness of the potentiometer housing and also by the inability to form large diameter wire mandrels into a small circle. The third alternative has a practical limit of accuracy dictated by the capability of winding machines. The fourth alternative is therefore the only one available and necessitates the use of exceedingly fine resistance wire, e.g., 0.40-4.00 mils when high resistance elements are required. This very small diameter resistance wire increases the end resistance problem since even a very short length of resistance wire between the fixed contact and the wiper positioned to obtain minimum resistance results in an appreciable resistance in series with the end terminal. By way of example, if the combined lengths of the two potentiometer end terminal resistance wires is one-eighth inch, the resistivity of the resistance wire is 800 ohm-circular mils/foot, and the resistance wire diameter is 0.50 mils, the end resistance of the potentiometer is 8.33 ohms. In many potentiometers, this amount of end resistance cannot be tolerated, even in such devices as trimmer potentiometers which ordinarily do not need to attain the accuracy of a precision-type potentiometer. This is so because the resistance of the trimmer potentiometer must usually be adjustable from 0 percent to percent of the total resistance of the potentiometer, an operation impossible to attain with a potentiometer having end resistance.

The use of very small diameter resistance wire also increases the problem of physically attaching fixed electrical contacts to the resistance element. Conventional welding techniques are unsatisfactory since the resistance wire is incapable of conducting the electrical current required for the welding operation. Conductive cements and solder fail to provide a bond satisfactory for potentiometers which must undergo high-temperatures or high-temperature variations.

It is an object of the present invention to provide an improved method of welding a fixed electrical contact to a resistance element.

Other and further objects, features and advantages of the invention will become apparent as the description proceeds.

Briefly, in accordance with a preferred form of the present invention, there is provided a potentiometer which comprises a housing, a resistance element fixedly mounted within the housing, and a wiper contact member engaging the resistance element and rotatably mounted with respect to the housing. Electrical output leads or terminals passing through the housing are connected to the respective ends of the resistance element by a fixed conductive element having a circular segment cross-sectional form. The planar face of this conductive member is welded to the resistance element so as to overlie the path described by the wiper contact member. The method for welding hereinafter described does not require the passage of welding current through any portion of the resistance element thus allowing the use of very small diameter resistance wire.

Fixed electrical contacts constructed according to the present invention reduce the end resistance to a negligible value since the movable wiper makes direct contact with the end termination. Moreover, the circular segment and other shapes hereinafter described minimize the mechanical resistance to the travel of the wiper contact in a continuous rotation type potentiometer.

A more thorough understanding of the invention may be obtained by study of the following detailed description taken in connection with the accompanying drawings in which:

FIG. I is a plan view of a miniature trimmer potentiometer embodying the invention in which two cutaway portions disclose parts within the potentiometer housing;

FIG. 2 is a perspective view of a resistance element having fixed electrical contact elements constructed in accordance with this invention;

FIG. 3 is an enlarged cross-sectional view taken along the lines 3-3 of FIG. 2;

FIG. 4 illustrates in cross section a prior art fixed electrical contact;

FIGS. 5a, 5b and 5c are cross-sectional views illustrating additional embodiments of fixed electrical contacts for variable resistances constructed in accordance with this invention; and

FIG. 6 is an enlarged cross-sectional view taken along the lines 6-6 of FIG. 2 illustrating the method for welding a fixed electrical contact to the resistance element of a variable resistance according to this invention.

Referring now to FIG. 1, there is shown a trimmer-type miniature potentiometer 10 having a substantially rectangular housing 11. This housing is shown considerably enlarged fonillustrative purposes, representative outside dimensions of such potentiometers being flinch X 96inch X 3/16 inch. Housing 11 includes an internal, cylindrically shaped cavity 12 and an additional cylindrical cavity 13 so located that the axes of the cavities 12 and 13 are mutually orthogonal. A worm gear or lead screw 14 is retained in the cavity 13 to allow teeth of the gear 14 to extend within cavity 12 for engaging the teeth of a wiper contact mounting gear 15. Within the cavity 13 is fixedly mounted the potentiometer resistance element 16 which, as shown in FIG. 2, is constructed of small diameter resistance wire 17 helically wound on a core or mandrel 18 of circular cross section and mounted so as to follow a circular path. The mandrel 18 is preferably a round copper wire which has been coated with an insulating enamel 19, such as polyvinyl formal polyester or phenolic-epoxy resin. it will be apparent that if the mandrel is constructed of a nonconductive material, the dielectric layer 57 may be deleted. The mandrel is normally straight when wound with the resistance wire 17 to a predetermined pitch and then formed into a helical configuration. This resistance element may be formed on a lathe or preferably according to the apparatus and methods taught and claimed by D. G. Marlowe in U.S. Pat. No. 2,334,880, entitled Apparatus for Winding Cores and H. H. Cary et al. in U.S. Pat. No. 2,620,790,entitled ?Method and Apparatus for Winding Resistance Elements," each of which is assigned to Beckman lnstruments, lnc. The helixed resistive element is then cut or sawed to produce one or more single-tum resistive elements of the appropriate length.

As shown in FIG. 1 and FIG. 2, fixed electrical contacts are attached to the respective ends of the resistance element and include respective conductive elements or tabs 25 and 26 which overlie the circular path described by the movable wiper. These conductive. elements are formed so as to have a circular segment cross sections of width W and height H. Conductive elements or tabs 25 and 26 are electrically connected to respective leads 27 and 28 which extend through the housing to the outside thereof. An additional lead 29 may, if desired, be attached to a'center tap (not shown) or other fixed taps of the resistance element 16.

A movable wiper contact 30 is fixedly mounted to the contact mounting gear [5. The cross-sectional shape of the movable wiper 30 is shown in FIG. 3 as a generally arcuately shaped member. This member is preferably dimensioned so as to make simultaneous contact with a minimum number of turns of the resistance element, e.g., two turns as shown in FIG. 3. Wiper contact 30 makes electrical contact with a slip ring (not shown) electrically connected to an externally extending electrical lead 31. The particular details of this and other structure of the potentiometer shown in FIG. 1 may be found in the copending application of Hans Habereder entitled Miniature Potentiometer, supra.

The fixed electrical contact elements 25 and 26 shown in H08. 1 and 2 provide a variable resistance having improved electrical linearity. Thus, it will be apparent that when the movable wiper is mechanically positioned to obtain the minimum resistance at the end of the resistance element being measured, it will abut one or the other of the conductive elements 25 and 26 thereby effectively reducing the end resistance to substantially -ohms resistance. An additional advantage of the fixed electrical contact structure shown is that it minimizes the jump-off" or resistance from the minimum resistance or mechanical end point to the electrical end point or wiper position where a measurable change in resistance is first observed with rotation of the wiper. By way of illustrating this advantage, a circular cross section conductor 35 is shown attached to the resistance element in FIG. 4. With this structure, it will be apparent that the electrical end point is displaced three or four resistance wire turns from the mechanical end point whereas, in the structure of this invention as shown in FIG. 3, the electrical end point is displaced one or at most two resistance wire turns from the mechanical end point.

A further advantage of fixed electrical contacts having a circular segment configuration is that the movable wiper contact will encounter substantially less mechanical resistance when traversing the fixed contact. In those potentiometers permitting a continuous rotation of the wiper or in those applications employing fixed electrical taps other than at the end terminations, the wiper must traverse its path relatively smoothly without encountering an obstacle sufficient to cause the wiper to become obstructed or physically deformed. The circular segment shown has been found to perform quite satisfactorily in miniaturized trimmer potentiometers.

Still another advantage of fixed electrical contacts constructed according to the present invention is that the broad width W of their planar face connection permits the fixed contact to engage several turns of the resistance wire thereby facilitating making an excellent mechanical and electrical connection to the resistance element.

Although the fixed electrical contacts have been described above as having a circular segment configuration, other crosssectional forms may be employed such as those shown in FIGS. 5a, 5b and 5c. For example, fixed electrical contact 40 has a triangular cross section as shown in FIG. 5a; fixed electrical contact 41 has a trapezoidal configuration as shown in H0. 5b, the longer parallel side 42 being affixed to the resistance element; and fixed electrical contact 43 has the form of a triangle having a rounded vertex as shown in FIG. 5c. in general, members whose cross sections are such that substantially all dimensions thereof parallel to the face affixed to the resistive elements are less than the dimension of the planar face are preferred for forming fixed electrical contacts for variable resistance elements. This configuration ensures that the wiper will encounter minimum mechanical resistance when traversing the fixed contact and moreover ensures that the jump-off resistance will be minimized.

A preferred method of welding fixed electrical contacts to the resistance element of a variable resistor is illustrated in FIG. 6. As shown therein, the fixed electrical contact element 26 is positioned on the resistance element 16 as illustrated. If the total ohmic resistance of the resistance element 16 is greater than the desired resistance of the finished variable resistance, the desired resistance may be obtained by appropriate positioning of the terminating electrical contacts 25 and 26. Similarly, intermediate taps may be positioned at the proper position for achieving the desired electrical characteristics. Spaced welding electrodes 45 and 46 are then brought into contact with the fixed electrical contact element 26 and a predetemiined pressure exerted between the electrodes and the resistance element. While this pressure is being exerted, an electrical current is caused to flow between the electrodes through a portion of the fixed electrical contact element 26. The resistance of the contact element between the terminals causes heat to be generated in the area between the welding electrodes and the combination of heat and pressure results in fusion between the fixed electrical contact element 26 and the turns of resistance wire 17 beneath the contact element.

The pressure and electric current settings used for the welding operation are determined by the resistivity and the size of the resistance wire used to form the resistance element and the size, material and shape of the fixed electrical contact element. The welding current must be such that the electrical contact element is not destroyed by excessive electrical current. However, sufficient electrical current must be applied to fuse the electrical contact and the resistance element. The following specific examples are given as representative of those used to form fixed electrical contacts according to this invention. ln each of these examples, the electrodes 45 and 46 were part of a Weldmatic welding head Model 1031 used in cooperation with a Weldmatic power supply Model 1016C manufactured by the Weldmatic Division of Unitec Corporation, Monrovia, California. In each example, the space between the welding electrodes remained constant at approximately 0.015 inch.

' 7 EXAMPLE 1 A high resistance element was formed using resistance wire having a diameter of 0.50 mils and comprising an alloy of 76 percent nickel, 20 percent chromium, 2 percent iron and 2 percent aluminum, this alloy having a resistivity of 800 ohmcircular mils per foot. The fixed electrical contact element was constructed of A nickel formed into a circular segment con figuration having a width W of 0.007 inch and a height H of 0.003 inch. For the welding operation, a light pressure was applied to the welding electrodes 45 and 46 and the current-time setting was 0.090watt-seconds.

EXAMPLE 2 A medium resistancelelement was formed using resistance wire having a diameter of 2.00 mils and constructed of an alloy of 57 percent copper and 43 percent nickel, this material having a resistivity of- 294 ohm-circular mils per foot. The fixed electrical contact element was constructed of A nickel fonned as a circular segment having a width W of 0.0l inch and a height 11 of 0.004 inch. A medium pressure was applied to the welding electrodes 45 and 46 and 0.090 watt-seconds used to form the weld-between the fixed contact element and the resistance element.

I EXAMPLE 3 A low resistance element was formed from resistance wire having a diameter of 4.00 mils and constructed of an alloy having 94 percent copper and 6 percent nickel, this alloy having a resistivity of 60 ohm-circular mils per foot. The fixed electrical contact element was formed of A nickel in the shape of a circular segment having a width W of 0.010 inch and a height H of 0.004 inch. Heavy pressure was used to apply the electrodes 45*and 46 to the fixed electrical contact element and 0.300 watt-seconds used to form the weld between the fixed contact element and the resistance element.

The welding procedure described hereinabove, has a significant advantage over the procedures used heretofore for attaching fixed electrical contacts to resistance elements since these prior methods require welding current to flow through the resistance element whereas in the method described herein, the welding current flows only through the fixedelectrical contact. Moreover, the method of this invention has been found to provide an excellent connection between the fixed contact element and the resistance element. Thus, these connections are characterized by high-physical strength, low

electrical resistance, and the ability to withstand high-temperatures and high-temperature variations.

Although the fixed contact structures and welding method for attaching the contacts are described above by reference to contacts which lie in the path described by the wiper contact, it will be apparent that the contact structures and method may be utilized for contacts outside this path. This may be necessary in some continuous. rotation potentiometers particularly those in applications wherein the wiper is driven at a relatively high velocity over the resistance element. Although end resistance will exist in such potentiometers, the other advantages of this invention as described above will be present.

Although exemplary embodiments of the invention have been disclosed and discussed, it will be understood that other applications of the invention are possible and that the embodiments disclosed may be subjected to various changes, modifications and substitutions without necessarily departing from the spirit of the invention.

We claim: l. The method of welding a fixed electrical contact to an electrical resistance element in the form of a plurality of turns of resistance wire wound upon and supported by a mandrel comprising;

positioning a contact element having a flat surface in engagement with atleast one of said turns of resistance wire of said resistance element' placing a pair of spaced welding electrodes in contact with a supplying electrical current to said electrodes, said current being sufficiently high to fuse said resistance element to said contact element but low enough to prevent damage to said contact element.

2. A method for welding a fixed electrical contact to a resistance element comprising a plurality of turns of very small diameter resistance wire wound upon and supported by a mandrel comprising the steps of:

positioning a contact element having a planar face so that a portion of said planar face engages one or more turns of said resistance wire;

placing a pair of spaced welding electrodes in contact with the portion of said contact element which engages said one or more turns of resistance wire; applying a predetermined pressure betweensaid resistance element and said welding electrodes; and

supplying an electrical current primarily through said electrodes and the portion of said contact element located between said electrodes, said current being sufficiently high to fuse the engaged turns of resistance wire to said contact element but low enough to prevent damage to said contact element.

3. A method of manufacturing electrical resistance elements comprising the steps of:

winding a plurality of tums of bare, resistance-type wire onto an insulated core, said wire having a predetermined melting point; fabricating a termination tab from a material having a melting point substantially higher than the melting point of said wire, said tab being of sufficient width to span a plurality of turns of said wire; placing said tab transversely across said core with a portion thereof in intimate contact with a plurality of turns of said wire; and I v supplying electrical energy to said tab by electrodes at opposite sides of said portion to heat said tab and fuse said wire onto said tab at said portion between said electrodes. 

1. The method of welding a fixed electrical contact to an electrical resistance element in the form of a plurality of turns of resistance wire wound upon and supported by a mandrel comprising; positioning a contact element having a flat surface in engagement with at least one of said turns of resistance wire of said resistance element; placing a pair of spaced welding electrodes in contact with a portion of said contact element which overlies said resistance element; and supplying electrical current to said electrodes, said current being sufficiently high to fuse said resistance element to said contact element but low enough to prevent damage to said contact element.
 1. The method of welding a fixed electrical contact to an electrical resistance element in the form of a plurality of turns of resistance wire wound upon and supported by a mandrel comprising; positioning a contact element having a flat surface in engagement with at least one of said turns of resistance wire of said resistance element; placing a pair of spaced welding electrodes in contact with a portion of said contact element which overlies said resistance element; and supplying electrical current to said electrodes, said current being sufficiently high to fuse said resistance element to said contact element but low enough to prevent damage to said contact element.
 2. A method for welding a fixed electrical contact to a resistance element comprising a plurality of turns of very small diameter resistance wire wound upon and supported by a mandrel comprising the steps of: positioning a contact element having a planar face so that a portion of said planar face engages one or more turns of said resistance wire; placing a pair of spaced welding electrodes in contact with the portion of said contact element which engages said one or more turns of resistance wire; applying a predetermined pressure between said resistance element and said welding electrodes; and supplying an electrical current primarily through said electrodes and the portion of said contact element located between said electrodes, said current being sufficiently high to fuse the engaged turns of resistance wire to said contact element but low enough to prevent damage to said contact element. 